Electrochemical cells in which a chemical reaction is forced by adding electrical energy are called electrolytic cells. Central to the operation of any cell is the occurrence of oxidation and reduction reactions that produce or consume electrons. These reactions take place at electrode/solution or electrode/gas phase interfaces, where the electrodes must be good electronic conductors. In operation, a cell is connected to an external load or to an external voltage source, and electrons transfer electric charge between the anode and the cathode through the external circuit. To complete the electric circuit through the cell, an additional mechanism must exist for internal charge transfer. This is provided by one or more electrolytes, which support charge transfer by ionic conduction, but they must be poor electronic conductors to prevent internal short-circuiting of the cell. A proton exchange membrane is a solid electrolyte that can be placed between the electrodes for internal charge transfer. One proton exchange membrane is sold under the name NAFION, a trademark of E. I. duPont de Nemours and Company of Wilmington, Del.
The simplest electrochemical cell consists of at least two electrodes and one or more electrolytes. The electrode at which the electron producing oxidation reaction occurs is the anode. The electrode at which an electron consuming reduction reaction occurs is called the cathode. The direction of the electron flow in the external circuit is always from the anode to the cathode.
Hydrogen and oxygen can be produced by electrolyzing water in a proton exchange membrane electrolyzer. In a water electrolyzer, water is introduced to the anode side of the proton exchange membrane and oxidized at the electrode surface. This reaction produces gaseous oxygen, which can be stored, and protons that pass through the proton exchange membrane to the cathode side. The electrons that were freed by the oxidation reaction are conducted to the cathode side through an external circuit by applied potential. The electrons and the protons recombine at the cathode electrode generating hydrogen by proton reduction.
Electrochemical cells that convert chemical energy into electrical energy are called fuel cells and are, by their operation, the opposite of electrolyzer cells. Fuel cells react different gases on anode and cathode electrodes having electrocatalytic surfaces that are positioned on opposite sides of an ion exchange membrane. Generally, the gas introduced to the anode is categorized a fuel while the gas introduced to the cathode is an oxidant.
In a hydrogen-oxygen fuel cell using a solid electrolyte, hydrogen is introduced via a gaseous stream to the anode side of a proton exchange membrane and oxygen is introduced via a second gaseous stream to the cathode side of the proton exchange membrane. In the fuel cell, the oxygen stream is electrochemically reduced and the hydrogen stream is electrochemically oxidized, the combined reactions producing water. The protons generated at the anode cross through the proton exchange membrane to the cathode, where they react in the oxygen reduction reaction with the electrons generated at the anode, making water. The electrons generated in the anode compartment are collected by a current collector and transported through an external circuit, which contains a load, to the cathode compartment.
Combining the functions of the proton exchange membrane electrolyzer and the proton exchange membrane fuel cell in the same device is described as a unitized regenerative fuel cell. In one design for a unitized regenerative hydrogen-oxygen fuel cell, each electrode is always in contact with the same gas, hydrogen or oxygen, and the electrical polarization of the stack is reversed when the system changes functions. Therefore, if the unit is operating as an electrolyzer, the oxygen electrode is the anode and the hydrogen electrode is the cathode. If the unit is operating as a fuel cell, the oxygen electrode is the cathode and the hydrogen electrode is the anode. It is therefore important that both electrodes be fabricated so that they do not degrade when operated in an oxidizing mode.
There are two problems encountered in designing electrodes for unitized regenerative fuel cell systems. First, electrodes suitable for use as oxygen evolution need to be hydrophilic to insure the presence of water at the electrode surface, but this type of electrode will readily flood during fuel cell operation. Second, electrodes suitable for use as oxygen reducers need to be hydrophobic to insure the presence of oxygen at the electrode surface, but they limit water availability at the electrode surface during the electrolyzer operation. Several methods have been demonstrated for overcoming this problem by using specially designed electrodes with complex multi-layer structures or membranes with internal fluid passages. While these methods overcome the basic problems, neither of these approaches leads to a stack with adequate performance for this application.
An important part of the development of an oxygen electrode is the choice of electrocatalysts. It is well known that the best electrocatalyst for oxygen reduction, platinum in its reduced form, is not the best catalyst for water oxidation and oxygen evolution. Other effective oxygen reduction electrocatalysts, such as chelated iron and cobalt, have the same limitation. Iridium oxide evolves oxygen at a far lower over potential than platinum and most other noble metals, and therefore, when used as the electrocatalyst, iridium oxide increases the efficiency of an electrolyzer. Over potential is defined as the amount by which the potential required to evolve oxygen from water exceeds the potential required for the ideal reversible reaction, 1.48 Volts at 25° C. This problem can be addressed by mixing the two catalysts, for example, platinum black for oxygen reduction and iridium oxide for oxygen evolution.
Additionally, an effective electrode has each of the catalyst particles in contact with at least one other electronically conducting particle so that it has a continuous electronic path to the electrical conducting current collector. It also has a continuous ionic network linking each catalyst particle to the membrane. Some researchers have developed complex arrangements with a variable internal structure to achieve these properties. It would be an advantage for an electrode to have a simple arrangement, with the same gross composition used throughout the volume of the electrode, making it simpler to fabricate.
In addition to selecting an electrocatalyst during the development of an oxygen electrode, it is necessary to develop a porous, conductive diffusion backing behind the electrocatalyst to insure the even delivery of reactants to and removal of products from the entire area of the electrode and to insure continuous electrical contact between the current collecting structure and the electrode. In a conventional fuel cell, this function is commonly carried out with a porous carbon structure. These are not suitable for long term use with an oxygen electrode used as an electrolyzer of water. Under oxygen evolution conditions, when water is being oxidized to produce oxygen, the carbon is subject to oxidation. While the oxidation rate is low, if used continuously over a period of time, enough carbon will be consumed to reduce electrical contact with the catalyst and thereby impair the functioning of the cell.
There is a need for an improved oxygen electrode that supports both the oxidation of water and the reduction of oxygen reactions. There is an additional need for an oxygen electrode having an electrocatalytic surface having reduced internal electrical resistance that is easily constructed. There is also a need for a diffusion backing for the electrocatalyst that is both electrically conductive and resistant to oxidation.